2 Sheds- at my unit SRG insist that if you issue a discrete transponder code to an aircraft to identify it, you MUST verify any associated mode C.
Towering Cu- it could be that RAF units don't even have access to local QNH as they tend to use QFE below TA and 1013 above it, hence for crossing the Brize Zone you would normally be asked to set Brize QFE, and outside the zone below TA you would be given the Cotswold regional; hopefully someone from Brize will pick this up and either confirm or refute!
I would personally suggest that if flying in class G below TA and NOT talking to ATC, you use RPS; if talking to ATC you use the observed QNH they give you.(But be warned some tower only ATC units plus FISO units with no qualified met observer may only be able to issue QFE!)